1. Field of the Invention
The present invention relates to photographic light-sensitive material, and more particularly to such a material which contains light-sensitive silver halide grains reduction-sensitized while being formed and each having a specified amount of silver iodide on its surface, and which has high sensitivity, and is low in fogging, and high in sensitivity of, particularly, spectrally sensitized regions.
2. Description of the Related Art
Basic properties required for a photographic silver halide emulsion are high sensitivity, low fog, and fine graininess.
In order to increase the sensitivity of an emulsion, (1) to increase the number of photons absorbed by a single grain; (2) to increase an efficiency of converting photoelectrons generated by light absorption into a silver cluster (latent image); and (3) to increase development activity for effectively utilizing the obtained latent image; are required. Increasing the grain size increases the number of photons absorbed by a single grain but degrades image quality. Increasing the development activity is an effective measure to increase the sensitivity. In the case of parallel development as color development, however, the graininess is generally degraded. In order to increase the sensitivity without graininess degradation, it is most preferable to increase the efficiency of converting photoelectrons into a latent image, i.e., increase a quantum sensitivity. In order to increase the quantum sensitivity, low-efficiency processes such as recombination and latent image dispersion must be minimized. It is known that a reduction sensitization which forms a small silver nucleus having no development activity inside, or on the surface of, a silver halide is effective to prevent recombination.
James et al. have found that the sensitivity can be increased with a lower fog level than that in normal reduction sensitization when kind of reduction sensitization, in which a coating film of an emulsion subjected to gold-plus-sulfur sensitization is vacuum-deaerated and then heat-treated in a hydrogen atmosphere, is performed. This sensitization method is well known as hydrogen sensitization and is effective as a lab-scale high sensitization means. The hydrogen sensitization is actually used in the field of astrograph.
The reduction sensitization has been studied for a long time. Carroll, Lowe et al., and Fallens et al. disclose that a tin compound, a polyamine compound, and a thiourea dioxide-based compound are effective as a reduction sensitizer in U.S. Pat. Nos. 2,487,850 and 2,512,925 and British Patent 789,823, respectively. Collier compares properties of silver nuclei formed by various reduction sensitization methods in "Photographic Science and Engineering", Vol. 23, P. 113 (1979). She used reduction sensitizations using dimethylamine borane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening. Various methods of reduction sensitization are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. Selection of a reduction sensitizer, and also a method of using a reducing agent are disclosed in, for example, JP-B 57-33572, JP-B 58-1410, and JP-A 57-179835. (Note: "JP-A" means Published Unexamined Japanese Patent Application, whereas "JP-B" means Published Examined Japanese Patent Application.) Techniques of improving storage stability of an emulsion subjected to reduction sensitization are disclosed in JP-A-57-82831 and JP-A-60-178445. Regardless of a number of studies as described above, an increase in sensitivity is insufficient as compared with that obtained in hydrogen sensitization in which a light-sensitive material is treated with hydrogen gas in a vacuum. This is reported by Moisar et al. in "Journal of Imaging Science", Vol. 29. P. 233 (1985).
Some of the publications specified above set out lists of the reduction sensitizers hitherto known. Among these sensitizers is ascorbic acid. In the publications, however, compounds such as thiourea dioxide are specified as preferable reduction sensitizers. In fact, reduction sensitizations using thiourea dioxide, silver ripening, and hydrazine are performed in the examples described in the publications. This fact suggests that ascorbic acid has not been regarded as preferable reduction sensitizers. The other reduction sensitization method is disclosed in JP- 57-179835.
To effectively utilize reduction sensitizations, the problem of storage stability of photosensitive materials must be solved. Techniques of improving the storage stability of a reduction-sensitized emulsion are disclosed in JP-A 57-82831 and JP-A 60-178445. These techniques, however, fail to provide sufficient storage stability.
Thus, there is also a demand to improve the storage stability of light-sensitive material which contains a reduction-sensitized emulsion.
The conventional techniques of reduction sensitization are insufficient to satisfy a recent demand for a photographic light-sensitive material with high sensitivity and high image quality. The hydrogen sensitization also has a drawback that a sensitizing effect is lost when a light-sensitive material is left in air after hydrogen sensitization. Therefore, it is difficult to utilize this sensitization method to prepare a photographic light-sensitive material for which no special apparatus can be used.
There is another problem related to the reduction sensitization. When color or spectral sensitization is performed, along with the reduction sensitization, the spectral sensitization reduce the increase of the sensitivity achieved by the reduction sensitization.